1. Field of the Invention
The present invention relates to a network relay device that relays data in a network.
2. Description of the Related Art
Networks generally adopt the policy of redundancy to prevent the unintentional stop of the functions in the whole network due to some failures occurring on lines and respective devices.
Layer 2 (data link layer) and Layer 3 (network layer) in an OSI reference model have some differences in network characteristics. In Internet Protocol (IP) as a typical example of Layer 3, data communication is disabled between devices interconnected via a line, unless IP addresses are allocated to the respective devices. In Ethernet (registered trademark) as a typical example of Layer 2, data communication is enabled between devices interconnected via a line.
A Layer 2 switch (hereafter referred to as L2 switch) is a typical example of a Layer 2 network relay device. Interconnection of L2 switches via multiple lines or adoption of the redundancy policy in multiple L2 switches may cause a loop in a network.
In packet transmission on the Ethernet (registered trademark), the L2 switch sends a packet having an unknown MAC address (unknown receiver) via all lines except a packet receiving line. The L2 switch is, however, incapable of storing the transmitted packet. The presence of a loop in the network causes endless circles of packet transmission and imposes a significant load on the packet transfer by the L2 switch. This may adversely affect other networks and occupy the circuit capacity.
A proposed countermeasure against this problem divides multiple lines and multiple L2 switches in the loop into the active system and the standby system. The standby system blocks the packet transfer by the L2 switch and sets the lines in logic disconnection, while only the L2 switch and the lines of the active system are used for data communication. This prevents a loop in the network.
The active system and the standby system are determined for each group of a single virtual LAN or of multiple virtual LANs. The L2 switch and the lines specified as the active system in one group of virtual LANs may thus be specified as the standby system in another group of virtual LANs, or vice versa.
The conventionally known structure uses a pair of L2 switches that are interlinked to switch over their operating statues between Master and Backup. This structure enable a quick switchover of the operation subject from the active system to the standby system in the event of some failure.
One of the two L2 switches is set in the Master operating status to be used as the active system (hereafter may be referred to as the Master switch), while the other L2 switch is set in the Backup operating status to be used as the standby system (hereafter may be referred to as the Backup switch). The Master switch opens ports and sets physically connected lines to logic connection to enable data communication. The Backup switch, on the other hand, blocks ports and sets physically connected lines to logic disconnection to disable data communication. During the operations, the Master switch and the Backup switch exchange control messages at regular intervals via a direct link to verify the mutual survivals. In the event of failure occurring in the Master switch or in any of the lines connected to the Master switch, the Backup switch detects the occurrence of the failure and switches over its operating status from Backup to Master. The Backup switch accordingly opens the blocked ports and changes the connection state of the physically connected lines to logic connection to enable data communication. The operation subject of the network is thus switched over from the active system to the standby system.
The relevant network technique is disclosed, for example, in U.S. Pat. No. 5,473,599.